With the recent spread of wireless communications systems for the remote control of the locking/unlocking of vehicle doors, there is a growing demand for inexpensive antenna devices which are securely controlled by vehicle drivers within a predetermined distance from the vehicles.
FIGS. 8 and 9 are perspective views of conventional antenna device 10. Antenna device 10 includes core portion 11 which is made of a square-column-shaped nickel ferrite magnetic material having a magnetic permeability of about 1500. Recess 12 is provided in an upper surface of core portion 11, and extends longitudinally throughout the entire length of the upper surface at substantially its center.
Antenna device 10 further includes coil 13 which is made of coiled copper wire coated with a heat resistant resin such as polyimide. Coil 13 is densely wound on the outer periphery of core portion 11 from the left to the right end of core portion 11.
Antenna device 10 further includes terminal base 14 which is made of a heat resistant resin, such as a liquid crystal polymer or polybutylene terephthalate. Terminal base 14 includes conductive chips 15, 16, 17, and 18, and terminal portions 19 and 20 which are planar and made of, e.g. copper alloy by insert-molding.
An end of the copper wire which forms coil 13 is connected by high-temperature soldering or caulking to an end of conductive chip 15 out of conductive chips 15, 16, 17, and 18 and terminal portions 19 and 20.
Conductive chip 15 and terminal portion 19 are formed unitarily.
The other end of the copper wire which forms coil 13 is connected to one end of conductive chip 16 by high-temperature soldering or caulking.
Conductive chip 17 is formed unitarily with conductive chip 16, and is exposed to terminal base 14.
Conductive chip 18 exposed to terminal base 14 is formed unitarily with terminal portion 20.
Capacitor 21 is mounted by cream soldering or other means onto conductive chips 17 and 18 exposed to terminal base 14.
Core portion 11 is fixedly fitted to terminal base 14 with an adhesive, such as silicon.
Antenna device 10 further includes auxiliary core 22 having a square column shape. Auxiliary core 22 is made of either a manganese ferrite magnetic material having a magnetic permeability of about 4000 or the same nickel ferrite magnetic material as core portion 11. Auxiliary core 22 is inserted into recess 12 from the right end of core portion 11, and is fixed to core portion 11 with an adhesive, such as silicon or a sealant.
FIG. 10 is a circuit diagram of antenna device 10. One end of coil 13 is connected to conductive chip 15 formed unitarily with terminal portion 19. The other end of the coil is connected to conductive chip 16.
Capacitor 21 is connected to conductive chip 17 formed unitarily with conductive chip 16, and to conductive chip 18 formed unitarily with terminal portion 20. Thus, capacitor 21 and coil 13 constitute a series resonant circuit.
In conventional antenna device 10, a resonance frequency of the antenna device is adjusted by movably inserting auxiliary core 22 into recess 12 of core portion 11 of coil 13. Antenna devices which have their resonant frequencies adjustable with an auxiliary core as in antenna device 10 are strongly demanded to reduce costs with their increasing installation rate in vehicles. In this type of antenna devices, the resonance frequency is adjusted within about 2% of the reference resonance frequency in terms of reliability and practicality.
An antenna device similar to conventional antenna device 10 is disclosed in Japanese Patent Unexamined Publication No. 2005-278130.